Heretofore known as refrigerating machine oils are mineral oils such as paraffin oils, naphthene oils and the like, and synthetic oils such as alkylbenzene oils, ester oils and the like. These oils are used chiefly for refrigerating machines using trichlorofluoromethane (R-11), dichlorodifluoromethane (R-12) or the like as the refrigerant. In recent years, however, it has been suggested that chlorine-containing fully halogenated hydrocarbons such as R-11 or R-12 released into the atmosphere would deplete the stratospheric ozone layer, thereby inflicting a serious adverse influence on the ecosystem including humans on the earth. Consequently a world-wide agreement, "The Montreal Protocol" signed in September 1987, calls for the restriction of consumption and production of the major ozone-depleting flons such as R-11 and R-12.
Use of CH.sub.2 FCF.sub.3 (R-134a) or like hydrogen-containing fluorinated hydrocarbons has been proposed as a substitute for R-11 or R-12. The proposed hydrocarbons are unlikely to deplete the ozone layer but very low in compatibility with conventional refrigerator oils. Because of this defect, the hydrogen-containing fluorinated hydrocarbon used as a refrigerant in a refrigerator together with a conventional refrigerator oil significantly reduces the refrigerating capacity and coefficient of performance.